1. Field
The present disclosure relates generally to inert gas welding. More particularly, the invention is directed to inflatable purge dams for retaining purge gas around a weld zone.
2. Description of the Prior Art
By way of background, inert gas welding is a species of arc welding in which the molten weld pool is shielded from atmospheric contamination and oxidation by bathing it with an inert gas, such as Argon, or a mixture of Helium and Argon. Popular examples of inert gas welding include TIG (Tungsten Inert Gas) welding and MIG (Metal Inert Gas) welding.
When welding together pipes and other enclosed structures using inert gas welding, it is important to purge the interior of the pipe or structure in the vicinity of the weld zone to prevent corrosion and the formation of oxides on the interior side of the weld pool. Purge dams are conventionally used for this purpose. For example, when butt-welding the ends of two pipe sections to form a consolidated pipe run, two purge dam structures are placed in the pipes, one in each pipe on either side of the weld zone. A purge gas can then be introduced into the area between the dams.
One well-known purging device used for pipe welding applications is the inflatable purge dam. As shown in FIG. 1, an inflatable purge dam typically include a pair of inflatable purge dam bladders 2 and 4 mounted on a purge gas delivery tube 6. The inflatable bladders usually comprise an inner bladder made from a resilient gas-impermeable material (such as latex rubber), and an outer protective cover made from fabric or other material (such as nylon) to protect the inner bladder from welding heat and caustic chemicals. The purge gas delivery tube is apertured inside the inflatable bladders so that the bladders will inflate when a purge gas “PG” is introduced. The purge gas delivery tube includes a pressure-sensitive gas discharge valve 8 located in the weld zone 10 that exists between the bladders. The discharge valve is designed to open when the purge gas reaches a predetermined pressure that is sufficient to inflate the bladders and ensure that the bladders form effective weld zone seals with the inside walls of the pipes to be welded. Once the weld zone seals have been formed, the discharge valve will open, thereby releasing purge gas into the weld zone. An air vent tube 12 typically extends through an outside portion of one of the bladders. The vent tube vents air from the weld zone as it is displaced by the purge gas, and may be connected to oxygen monitoring equipment for monitoring the oxygen content within the weld zone.
Conventional inflatable purge dam devices as described above are designed for use with pipe installations having a specific nominal pipe size (NPS). Although such devices may work satisfactorily with several different pipe schedules within a single NPS (e.g., by relaxing the purge gas pressure to partially deflate the purge bladders as the pipe wall thickness increases and the inside diameter decreases), they are not intended to work over a range of NPSes. For example, applicants have observed that a conventional inflatable purge dam designed for pipes having an NPS of 3 inches may well be able to handle pipe schedules 10, 40, 60 and 80, but will not be usable with pipes having an NPS of 2 inches or 4 inches. Applicants have further observed that conventional inflatable purge dams as described above may allow oxygen leakage into the weld zone around the outside of the inflatable bladder that carries the vent tube 12. It appears to applicants that this is due to the vent tube rippling or otherwise distorting the outer surface of the bladder where it seals against the inside pipe wall.
Accordingly, applicants submit that there is presently a need for improvement in the construction of inflatable purge dam. What is required in particular is an inflatable purge dam apparatus that can be used with a range of pipe sizes and/or does not leak in the vicinity of a vent tube.